When a vertebra is broken or crushed it is frequently necessary to ablate the body of the crushed or broken vertebra or vertebrae, normally along with the flanking disks. In order, however, to prevent the spinal column from collapsing with damage to the fragile spinal cord running in the vertebral foramen forward of the vertebral body, it is necessary to employ an implanted spacer. This device is braced vertically between the bodies of the adjacent vertebra and holds them apart at the desired spacing. It may even serve to distract two vertebrae which have become too closely spaced due to crushing of a vertebra or disk.
To this end as described in US 2004/0049271, US 2005/0113921, U.S. Pat. No. 6,015,436, and CA 2,216,450 the implant has two parts that can be moved and locked relative to each other along an axis extending between the two disks. This way the implant can be shortened and fitted in place by the surgeon, then extended to lock in place and even distract the adjacent vertebrae.
Frequently after insertion the implant is intended to become fused in place, creating a region including itself and the two adjacent vertebra where the spine is completely rigid. This constitutes a disability that is normally considered more desirable than the condition the implant was inserted to treat.
It has been suggested, however, in US 2004/0210312 and 2005/0060036 to provide some degree of movement in the implant. This eliminates the complete rigidity otherwise produced, but such implants are complex to install and not normally considered a safe treatment for an active person.